1. Field of the Invention
The present invention relates to a switching power supply circuit that is used for electronic devices such as an LED driver and the like, and more particularly, to a switching power supply circuit having a function of performing a constant current control on output current.
2. Description of the Related Art
Conventionally, a switching power supply circuit is known as a small, light and high-efficient power supply circuit and is used as a power source for various electronic devices. In general, the switching power supply circuit is often used as a constant voltage source that supplies a constant voltage to a load. However, the switching power supply circuit is also used as a constant current source that supplies constant current to a load (for example, refer to JP-A-2009-148107).
In a switching power supply device shown in FIG. 1 of JP-A-2009-148107, a main switching element is used as a so-called high-side switching element that is provided at a positive terminal side of an input power source. In this device, a current detection resistance is serially connected to a load to directly detect output current that flows in the load. Thus, it is possible to perform a constant current control relatively easily. However, since the main switching element is provided at the high side, a driving circuit of the switching element is relatively expensive.
In view of the problem, it is suggested an illumination device using a switching power supply circuit in which a main switching element is used as a so-called low-side switching element that is provided at a negative terminal side of an input power source (for example, refer to JP-A-2009-134946).
In JP-A-2009-134946, a switching power supply circuit unit including a voltage dropping chopper circuit is switching-controlled by a control circuit IC that also serves as a switching element and performs a current limit control that turns off the switching element when a peak value of current flowing in the switching element exceeds a threshold current.
However, the switching power supply circuit unit described in JP-A-2009-134946 has following problems.
That is, in the switching power supply circuit unit, when an input voltage is denoted as Vin, an output voltage is denoted as Vout, an inductance value (L value) of a choke coil is denoted as L1 and a switching period of the switching element is denoted as Ts, a relationship between output current Io and threshold current Ip is expressed by an equation (1).Io=Ip−(½)×((Vin−Vout)/L1)×(Vout/Vin)×Ts  (1)
As can be seen from the equation (1), when the inductance value (L1) of the choke coil is sufficiently high, the output current Io becomes approximately same as the threshold current Ip. However, when the inductance value (L1) of the choke coil is increased, a size of the choke coil becomes larger and more expensive. Thus, there is a limit to an increase in the inductance value (L1) of the choke coil. Therefore, the output current Io is changed as the input voltage Vin or output voltage Vout changes.